FIGS. 1A and 1B illustrate a typical method of manufacturing a double heterojunction LED of the prior art and the structure of such a prior art LED. Turning first to FIG. 1A, a substrate 100 of GaAs is doped to have a p+ type conductivity. On a major surface of substrate 100 an epitaxial layer 101 of GaAs, doped to have an n type conductivity, is formed. A masking layer 102 is then formed on epitaxial layer 101. Typically masking layer 102 is formed by placing a uniform layer of a photosensitive resist material on epitaxial layer 101. Photoresist 102 may then be exposed and developed leaving the resist material covering those regions of epitaxial layer 101 to be retained while leaving exposed those regions of epitaxial layer 101 which are to be removed by etching. The assembly is then subjected to an etching step which removes unprotected portions of epitaxial layer 101. Following the etching step resist layer 102 is completely removed and the exposed surface of epitaxial layer 101 carefully cleaned to remove the resist material and the etchant.
Turning now to FIG. 1B an epitaxial layer of Al.sub.x Ga.sub.1-x As doped to have a p type conductivity 103 is formed on the exposed surface of n type epitaxial layer 10. When epitaxial layer 103 is grown the gap formed in epitaxial layer 101 by etching will be filled creating p type region 104. The upper surface 105 of epitaxial layer 103 will be substantially, but not completely, planar. In particular, in the region directly over region 104 a small depression will exist due to incomplete filling of region 104.
Another p type epitaxial layer 106 Al.sub.x Ga.sub.1-x As is then formed on surface 105 of epitaxial layer 103. Epitaxial layer 106 will form the active layer of the LED when construction of the LED is completed. In particular, region 107, indicated by crosshatching, forms the active region of the device. The crosshatching is intended to show where the recombination, and thus light production, will take place, but does not indicate any difference in doping levels between the active region 107 and the rest of active layer 106.
An n type epitaxial layer 108 of Al.sub.x Ga.sub.1-x As is formed over layer 107. An n type epitaxial layer 109 of GaAs is formed over layer 108. A portion of epitaxial layer 109 directly over active region 107 is etched away, resulting in window 109, in order to allow light produced in active region 107 to escape.
In operation, epitaxial layer 109 serves as a capping layer and is primarily provided to allow good electrical contact with other circuitry. Epitaxial layers 108 and 103 each serve as confining layers, meaning that they tend to hold charge carriers in active layer 106 until recombination occurs. Epitaxial layer 106 acts as the active layer as described above. Epitaxial layer 101 is commonly known as a blocking layer because, during normal operation of the device, a reverse biased p-n junction exists between regions 101 and 103. Thus, current flowing from region 103 to substrate 100 must flow through region 104. It is this limitation of current flow to region 104 that limits the active region to region 107 of active layer 106. Substrate 100 acts to provide electrical contact to other circuitry and to provide structural integrity to the device.
The effective modulation bandwidth of the prior art LED shown in FIG. 1 is limited by the series resistance presented by regions 100 and 104 and by the minority carrier lifetime in active region 107. The minority carrier lifetime may be reduced, thus increasing the modulation bandwidth of the device, by either increasing the doping level in active region 107 or by reducing the size of active region 107. If the doping level in active region 107 is increased to too high of a level an excessive number of non-radiative centers are formed therein and the increase in modulation bandwidth is accompanied by a decrease in quantum efficiency. Therefore, a preferable way of decreasing minority carrier lifetime in active region 107, and thus increasing the modulation bandwidth of the device, would be to reduce the size of active region 107.
A second difficulty with the prior art process as described above is that both the thickness of n type epitaxial layer 101 and the etching to form region 104 must be very carefully controlled. If the etching does not form a gap all of the way through layer 101 a reverse biased p-n junction will be formed between region 104 and region 101, at worst preventing effective current flow between region 103 and region 100 and at best substantially increasing the series resistance of the device. Clearly this would prevent proper operation of the LED. Alternatively, if the well etched to form region 104 is very deep, p type epitaxial layer 103 must be very thick in order to insure that region 104 will be completely filled. If epitaxial layer 103 is thick the series resistance of the LED will be substantially increased. In order to keep the well depth as shallow as possible, layer 101 is typically in the range of two to three micron thick. Smaller dimensions for layer 101 may be insufficient to perform the required current blocking. Because etching proceeds laterally as well as vertically the minimum thickness for layer 101 tends to set a minimum width for region 104. Additionally if the aspect ratio, i.e. ratio of the height to the width, of an etched well is too great, liquid phase epitaxial growth in that well will be difficult. This phenomenon further limits the minimum width of region 104 and hence of active region 107. At the same time the fact that epitaxial layer 101 is only two to three microns thick prevents a thorough cleaning job on the surface of that region subsequent to the removal of resist mask 102 but prior to growth of subsequent epitaxial layers because cleaning could damage such a thin layer. Various contaminants left on the surface of region 101 may have a deleterious effect on the quality of epitaxial layers grown thereon.